Compressors and more specifically centrifugal compressors, which are utilized as for example in providing the atomizing air to a spray painting system have a filtered inlet air supply. The ambient air from the room, which enters the inlet plenum of the compressor, is filtered by a relatively low pressure drop filter of the replaceable type. A filter is necessary for the protection of the internals of the compressor from damage due to dirt and particles that are present in the ambient air drawn into the inlet plenum and compressor assembly. These particles of dirt and foreign matter, instead of damaging the internals of the compressor, tend to plug the filter. In time, the differential pressure across the filter increases and the compressor is required to work harder in order to draw the required amount of air through the filter and into the inlet plenum. This occurs because a centrifugal compressor is inherently a constant mass machine, meaning that the mass flow rate of air (pounds per minute) remains constant in most operating conditions. Therefore, the compressor must generate the differential pressure required to draw the constant mass of air into its inlet plenum. This compensating action of the compressor itself leads to a very low, subatmospheric, pressure in the inlet plenum when the ambient air being drawn through the filter is at atmospheric pressure. Such a low pressure in the inlet plenum, when compared to the relatively higher pressure at the last stage of the compressor at the outlet, means that there is a constant force on the internals of the compressor acting from the inlet toward the outlet causing flexure of the successive stages of the centrifugal compressor. Such flexure causes increased wear on the internals of the compressor leading to the potential for failure of the compressor due to mechanical fatigue of the materials of the construction.
Additionally, it may be noted that such a differential pressure between the inlet plenum and the outlet plenum occurs even if there is no filter present or the filter is always changed in a timely manner. The results of even the normal differential pressure across the turbine internals leads to some material fatigue, but additionally leads to the use or input of more energy at the compressor motor than would be required if the differential pressure from inlet to outlet were even further reduced. Since the reduction of the outlet pressure is an undesirable solution to the problem, it would be most desirable to moderately increase the pressure at the inlet plenum without any corresponding changes in the outlet pressure thereby achieving a lower differential pressure from the inlet plenum to the outlet plenum of the compressor and thereby conserving energy.
Therefore, there is a need for a method of protecting a compressor from undue differential pressures from the inlet plenum to the outlet plenum, especially those higher differential pressures caused by a dirty filter. Additionally, there is a need for a method of altering the pressure at the inlet plenum to produce a corresponding reduction in the energy input to the compressor that is required to maintain a constant outlet pressure at a constant mass flow.